With the help of maximum ratio transmission [1]}, the optimal beamforming vector can be expressed as \( {{\bf {w}}_k} = \frac{{{{\bf {h}}_k}^T_1}}{{\left\Vert  {{{\bf {h}}_k}} \right\Vert }} \) . Thus, we have \({\left\Vert  {{{\bf {h}}_k}{{\bf {w}}_k}} \right\Vert ^2} = \sum \limits _{j = 1}^{{N_T}} {h_{k,j}^2}\) .
